Calculation of the Spin Susceptibility of Disordered Binary Alloys: Application to Pt-Pd, Rh-Pd, Ni-Rh, and Ni-Pd

Recently the spin susceptibility χ of a one-band model for a disordered binary alloy with intraatomic Coulomb interactions and short-range scattering potentials was calculated using the coherent-potential approximation (CPA). The formal expression for χ, which is applicable to alloys of arbitrary concentration and potential-scattering strengths, was found to reduce to previously obtained expressions for the susceptibility in the dilute-alloy limit and to contain as a special case the uniform-enhancement model for χ. In the present paper this theory for the spin susceptibility is applied to several binary Ni, Rh, and Pd alloys. Good agreement with experiment is obtained for χ as a function of x in PtₓPd_1-x, RhₓPd_1-x, NiₓRh_1-x, and NiₓPd_1-x when a simple "steeple model" for the density of d-electron states is used. The sign of the potential-scattering parameter is obtained from renormalized-atom calculations; its magnitude is allowed to vary arbitrarily. Good agreement between theory and experiment makes it possible to determine the physical mechanisms which govern the behavior of χ in the four alloy systems considered. It is shown that potential-scattering effects which change the density of states at the Fermi energy in the alloy from the value in the pure crystals must be included in calculations of χ in PtₓPd_1-x. A uniform-enhancement model, in which the alloy is replaced by a periodic crystal at each site of which the Coulomb interaction energy is given by the average of the intra-atomic Coulomb energies, is found to approximate the calculated spin susceptibility in PtₓPd_1-x to within an accuracy of 10%. For RhₓPd_1-x alloys it is concluded that it is more likely that the nonmonotonic x dependence of χ is due to a relatively large contribution to the susceptibility associated with Rh sites than to a rigid-band density-of-states effect. This conclusion is in agreement with recent NMR data. The theoretically determined spin susceptibility in NiₓRh_1-x alloys for $x<~0.50$ may be approximated to within an accuracy of 10% by a uniform-enhancement model, providing the density of states at the Fermi energy is calculated self-consistently at each concentration x using the CPA. Thus both the Ni and Rh atoms may be viewed as participating equally in the ferromagnetic phase transition in NiₓRh_1-x which takes place for $x>~0.63$. By contrast, for NiₓPd_1-x alloys in which it is found that the Ni sites make a relatively large contribution to the susceptibility, the Ni atoms appear to be mainly responsible for the ferromagnetic phase transition which occurs at very low Ni concentrations $x>~0.022$.